PROJECT SUMMARY In a few months, a potential second wave of CoVID-19 will be superimposed on the influenza season, which starts in the fall and typically peaks between December and February in the United States. By then, the majority of Americans are unlikely to be immune to SARS-CoV-2. An effective SARS-CoV-2 vaccine is likely to take months to years to achieve widespread protection. In addition, reduced social distancing is likely to elicit regional surges in CoVID-19. Potential synergy between these two respiratory pathogens could result in significant morbidity in the short-term. In the long term, we face the reality that SARS-CoV-2 may assume endemic status and may interact with other seasonal respiratory pathogens for the foreseeable future. The overall goal of this proposal is to determine whether prior influenza infection worsens CoVID-19- like disease in the SARS-CoV-2 Syrian hamster model. Recent studies indicate that ACE2 may be upregulated by influenza infection. The Syrian hamster supports infection with human influenza A H1N1 subtypes as well as contemporary H3N2 subtypes that cannot replicate in mice. Additionally, the Syrian hamster is a faithful spontaneous animal model of CoVID-19. We will pursue two aims: 1) to characterize interferon-driven immune responses and ACE2 expression in hamsters infected intranasally with contemporary H3N2 and H1N1 influenza strains and 2) to characterize clinical disease course, immune responses and translationally relevant biomarker alterations following SARS-CoV-2 infection of acutely infected and recovered influenza-infected hamsters. We will use a factorial design to assess the effects of controllable variables (influenza virus infection alone, combined influenza/ SARS-CoV-2, acute/recovered status and sex) on clinically relevant outcome measures (body weight, duration/severity of clinical illness and pulmonary injury scoring). Cytokine, immune and ACE2 responses will allow us to assess association of these variables with infection status and clinical phenotype. These approaches will provide direct and translationally relevant data regarding impact of influenza on SARS-CoV-2 clinical phenotype, as well as advance understanding of underlying immune responses.